The invention relates to a process for treating a gas turbine part which has been coated with a ceramic protective layer and to a coated gas turbine part.
It is generally known from numerous documents to provide turbine blades or vanes, i.e. guide vanes or rotor blades of gas turbines, with one or more protective layers in order to protect the turbine blade or vane from the thermal and mechanical loads, oxidation and other harmful influences which occur during operation and to extend the service life of the turbine blade or vane in this way. A first protective layer on the turbine blade or vane generally consists of a metallic alloy, such as MCrAlY, where M represents Ni, Co or Fe. This type of metallic coating is used to protect against oxidation. A second, rougher coating comprising MCrAlY is applied to the first layer using different coating parameters. This layer is also known as a bond coating. Coatings of this type are known from numerous documents in the prior art, for example from U.S. Pat. No. 3,528,861 or U.S. Pat. No. 4,585,481.
Moreover, a further protective layer of TBC (Thermal Barrier Coating), which consists of a ceramic material (Y-stabilized Zr oxide) and is used as thermal protection, is applied. Ceramic coatings and coating methods are known, for example, from the documents EP-A2-441 095, EP-A1-937,787, U.S. Pat. Nos. 5,972,424, 4,055,705, 4,248,940, 4,321,311, 4,676,994, 5,894,053. The applied protective layers generally have a relatively high surface roughness. However, this surface roughness has a positive influence on the heat transfer, so that increasing roughness increases the thermal load on the base material. To avoid this, a process for smoothing the surface is known, for example, from EP-A2-1 088 908. On the other hand, however, a ground surface has an adverse effect on the flow characteristics and in particular the detachment characteristics.
It is an object of the invention to provide a process which allows the heat transfer to the hot gas from a gas turbine part which is coated with a ceramic protective layer around which a hot gas flows to be reduced, so that improved protection of the base material of the gas turbine part is achieved. At the same time, the flow characteristics around the gas turbine part and therefore the efficiency of the overall installation are to be positively influenced. A further object is to produce a corresponding gas turbine part using this process.
According to the invention, in a process as described herein, this object is achieved by the fact that the roughness of the ceramic layer which has already been applied to the base material is reduced at at least one first location, and the original roughness of the ceramic layer is retained at at least one second location.
The invention also consists in a gas turbine part which is produced using the process according to the invention, in which the roughness of the ceramic protective layer is reduced compared to the original average roughness at at least one first location on the surface, and the original roughness of the ceramic protective layer is retained at at least one second location on the surface.
In principle, it is possible to reduce the roughness by grinding, sand-blasting, polishing, smoothing, brushing or in other suitable ways which are known from the prior art.
In a particular embodiment, the gas turbine part is a turbine blade or vane which is coated with Y-stabilized Zr oxide.
To positively influence the detachment characteristic at the surface of the turbine blade or vane, the roughness can be retained only at at least one location of the turbine blade or vane which is remote from the flow, while the remaining surface area of the turbine blade or vane is ground smooth. In this way, the heat transfer at the parts of the surface which have been ground smooth is advantageously reduced, so that the heat transfer deteriorates at these locations and the cooling of the base material is improved for the same cooling capacity. However, at locations at which there is a risk of flow detachment, the ceramic protective layer remains rough, so that at these locations a certain turbulence is generated and the flow remains in place for a longer time. These simple measures advantageously increase the efficiency of the entire installation.